1. Centre de Pédologie Biologique, CNRS, FRE 2111 associated with the H. Poincaré University, 17 rue N. D. des Pauvres, B.P. 5, F-54501 Vandoeuvre-les-Nancy Cédex, France, Department of Ecology, Royal Veterinary and Agricultural University, Thorvaldsensvej 40, DK-1871 Fredriksberg C, Denmark, and Institute for Agrobiotechnology, Konrad Lorenz Strasse 20, A-3430 Tulln, Austria